The present invention relates to a continuous catalytic hydrogenation process, in which the hydrogenation catalyst suspended in the reaction mixture is recirculated. This process is particularly suitable for the hydrogenation step in the anthraquinone process for the production of hydrogen peroxide.
In known hydrogenation processes the catalyst is generally used as a fixed bed, as a supported or as a suspended catalyst.
In the fixed bed reactor the catalyst is applied to a support. A great disadvantage of this type of reactor is the slow transfer of the gaseous hydrogen into the solution and on to the catalyst surface.
Various ways of bringing the hydrogen into solution more rapidly have been suggested. U.S. Pat. No. 2,837,411 describes the saturation of the solution with hydrogen before hydrogenation in a separate tank. Considerable investment costs and the fact that only a fraction of the hydrogen required in the reaction can be brought into solution make this route unattractive.
Even the pre-mixing of the hydrogen with solution to be hydrogenated by means of a static mixer, as described in U.S. Pat. No. 4,428,922, leads only to a slight improvement, since the hydrogen bubbles coalesce on contact with the inserts in the reactor.
When using a supported or suspended catalyst, as described in EP-A-111 133, these disadvantages do not occur.
In EP-A-111 133 it is shown that in a continuous co-current catalytic hydrogenation process for the production of hydrogen peroxide by the so-called anthraquinone process, the efficiency of the hydrogenation can be increased if this is carried out in a loop reactor at flow velocities of more than 3 m/s, preferably 4-7 m/s. The flow velocity is selected such that the gas/liquid interface remains as produced at the beginning of the reactor. In this way the hydrogen can react completely along the reaction zone and no separations occur between the gas and the liquid phase.
It is disadvantageous in this process described in EP-A-111 133 that the mixing devices for the substance to be hydrogenated and the hydrogen lead to large pressure drops, particularly when tubular reactors are to be operated at the high circulating flows and circulation rates specified, and therefore a higher input of energy is necessary.
An object of the present invention was therefore to overcome the problems associated with prior known processes and thereby increase the efficiency of a hydrogenation process in which a reaction mixture containing the substance to be hydrogenated, the hydrogenation product, hydrogen and the catalyst suspended in the reaction mixture, is recirculated.